1. Field of the Invention
This invention relates to an electro-conductive liquid crystal element to be applied to electronic devices and an organic electro-luminescence element (to be referred to as xe2x80x9corganic EL elementxe2x80x9d herein after) utilizing such a liquid crystal element.
2. Related Background Art
Of organic EL elements, carrier-injection type EL elements were studied in-depth particularly in terms of organic solids such as anthracene single crystal in the 1960s. Such elements were of the single layer type. Thereafter, Tang et al. proposed multilayer type organic EL elements comprising a luminescent layer and a hole transport layer between a hole injection electrode and an electron injection electrode. Injection type EL elements commonly have a light emitting mechanism including stages of:
electron injection from the cathode and hole injection from the anode;
movement of electrons and holes in solid;
recombination of electrons and holes; and
light emission from singlet excitons.
A typical multilayer type organic EL element is prepared by forming an ITO film on a glass substrate as anode, forming thereon TPD (N,N-diphenyl-N,N-di(3-methylphenyl)-1,1-biphenyl-4,4-diamine) to a thickness of about 50 nm, forming thereon Alq3 (tris-(8-quinolaritho)aluminum) to a thickness of about 50 nm and depositing an Alxe2x80x94Li alloy thereon by evaporation as cathode. Injection of holes to the TPD can be facilitated when 4.4 to 5.0 eV is selected for the work function of the ITO used for the anode, whereas a metal material that is stable and has a small work function is selected for the cathode. Typically, an alloy of Al and Li or Mg and Ag is used. With this arrangement, the element emits green light when a DC voltage between 5 and 10V is applied thereto.
It is also known to use electro-conductive liquid crystal for the carrier transport layer. For example, xe2x80x9cNaturexe2x80x9d, Vol. 371, P. 141, D. Adam et al. reports that the mobility of discotic liquid crystal that is a long chain triphenylene type compound is between 10xe2x88x923 and 10xe2x88x922 cm2/Vsec in the liquid crystal phase (Dh phase) and 10xe2x88x921 cm2/Vsec in the mesophase (intermediary phasexe2x80x94not liquid crystal phase). In the case of rod-shaped liquid crystal, xe2x80x9cApplied Physicsxe2x80x9d, Vol. 68 [1], Hanna, J., p. 26 reports that the mobility of phenylnaphthalene type smectic B phase is 10xe2x88x923 cm2/Vsec or more.
xe2x80x9cLiquid Crystalsxe2x80x9d, 1997, Vol. 23 [4] INGAH. STAPFF et al. pp. 613-617 reports an organic EL element using triphenylene type discotic liquid crystal that is designed in an attempt for using such liquid crystal for electro-luminescence. xe2x80x9cPOLYMERS FOR ADVANCED TECHNOLOGIESxe2x80x9d, 1998, Vol. 19, pp. 443-460 and xe2x80x9cADVANCED MATERIALSxe2x80x9d, 1997, Vol. 19 [1], p. 48 also report the use of liquid crystal for electroluminescence.
Meanwhile, Friend et al. of Cambridge University reported organic EL elements (polymer type EL elements) realized by using a polymer material in 1990. It may be safe to say that the mechanism of operation of these polymer type EL elements is essentially the same as that of known EL elements made of a low molecular weight material (low molecular weight type EL elements), the difference being only in the molecular weight of the organic material and the film forming method. Polymer type EL elements can be prepared by using a wet film forming method such as spinner coating. While polymer type EL elements are inferior to low molecular weight type EL elements including those proposed by Tang et al. in terms of the intensity and efficiency of light emission, unlike the latter, they provide advantages such that it is possible to use a wet process for preparing them. Materials that can be used for polymer type EL elements include conjugated polymers such as polyparaphenylene-vinylene (PPV) and polyfluorene, soluble poly-2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene (MEH-PPV) that is produced by introducing substituents to PPV, as well as polyvinylcarbazole (PVK) having a structural formula as shown below whose side chains are active and show a high carrier transport characteristic although its main chain is electrically inactive. 
As pointed out above, known polymer type EL elements are inferior to low molecular weight type EL elements in terms of the intensity and efficiency of light emission. However, a method of improving the luminance and the efficiency of light emission by replacing the aluminum of the cathode by calcium is known (D. Braun, A. J. Heeger, Appl. Phys. Lett., 58, 1982, 1991). When the aluminum of the cathode is replaced by calcium, the work function falls from 4.2 eV to 3.0 eV to make it easy to inject electrons. Metals such as calcium whose work function is low are reducing and highly reactive so that the calcium in the polymer shows a state of being polymer-doped at and near the interface and hence it may be safe to assume that the electron barrier is made low (J. M. Bharathan, Y. Yang, Appl. Phys. Lett., 72, 2689, 1998). However, a calcium electrode is very unstable and its service life is extremely short.
There is known a method of injecting holes from the anode that utilizes an electro-conductive polymer such as polyaniline or polythiophene as a buffer layer for improving the injection efficiency. However, the anode is normally made of ITO and its surface shows undulations. Therefore, the technique of forming a polymer film on the ITO electrode does not give rise to a good result in terms of adhesiveness and bonded area.
As described above, while polymer type EL elements provide advantages relative to low molecular weight type EL element in terms of manufacturing process, neither the interface with the cathode nor that with the anode allows carriers to be injected satisfactorily through them partly because of the strong film forming characteristic of the polymer material. As a result, polymer type EL elements are inferior to low molecular weight EL elements in terms of the stability (service life) of elements and the efficiency and intensity of light emission.
The electronic structure of organic EL elements is such that no thermally excited free carriers exist because of the energy gap is as large as about 3 eV and the carriers injected from the electrodes form an electric current (space charge limiting current). Therefore, the efficiency of injecting carriers from the electrodes is a serious problem. If the injection efficiency is low, a high voltage needs to be applied in order to provide a necessary amount of electric current and thereby the thickness of layers of the element needs to be reduced. Then, a short-circuiting can occur between the electrodes and the capacitive load of the element has to be increased.
In view of the above circumstances, it is therefore the object of the present invention to provide an organic EL element of the polymer type that is advantageous in terms of manufacturing process and operates as excellently as low molecular weight type EL elements in terms of the stability and the efficiency and intensity of light emission.
According to the invention, the above object is achieved by providing an electro-conductive liquid crystal element comprising a pair of oppositely disposed electrodes, an electro-conductive liquid crystal layer formed so as to contact at least one of the electrodes, said electro-conductive liquid crystal layer having in the molecular structure thereof an electron resonance structure and a temperature region showing a liquid crystal phase, and an electro-conductive polymer layer formed so as to contact the electro-conductive liquid crystal layer.